This invention relates generally to voltage-to-frequency converters and more particularly to temperature compensated, switchable current sources used in such converters.
As is known in the art, voltge-to-frequency converters have been used for many years in a wide variety of applications. One type of voltage-to-frequency converter produces a series of pulses of fixed width and height at a rate proportional to the level of an input voltage. In such converter the input voltage is passed through a resistor-capacitor integrator to one input of a voltage comparator. The output of the voltage comparator is coupled to a one shot multivibrator, the output of which is fed back to the first mentioned input of the voltage comparator. The voltage at the output of the integrator increases, in response to the input voltage, at a rate proportional to the level of the input voltage. As soon as the integrator output exceeds a fixed, predetermined voltage level which is established by a voltage source coupled to the other input of the voltage comparator, the voltage comparator triggers the one shot multivibrator, the latter thereby producing a pulse of fixed time duration. In the ideal case this pulse instantaneously resets the voltage stored in the capacitor of the integrator to zero volts. The process continues and it follows that the rate at which the pulses are produced by the one shot multivibrator will be, to an approximation, proportional to the charge rate of the capacitor and therefore, to an approximation, to the level of the input voltage.
While the voltage-to-frequency converter described above may be adequate in some applications, such converter has been modified for other applications to include the use of a charge dispensing technique to improve the linearity of the converter. With such technique the charge stored in a "timing" capacitor is quickly dumped into a large discharge capacitor. The charge is generally supplied from a switchable current source. In order for such converter to find wide application, it is generally required that such current source operate over a wide range of temperature without appreciable variation in the amount of current supplied to the capacitor. Also, where a switching arrangement is used, it is generally necessary that the action of such switching arrangement be quick and precise in order to insure accurate voltage-to-frequency conversion.